When a heating element is secured to a base by tabs and brought to radiance for heating purposes by passing an electric current through it, there is a disadvantage in that the strip glows less brightly in the regions adjacent to the tabs. The reason for this is two-fold. Firstly, heat is conducted from the strip into the mounting tabs and then into the base. Thermal conduction into the base can be reduced somewhat by employing a base with very good thermal insulation properties, a particular example of such being microporous thermal insulation material. Secondly, it is found that the electric current density in the strip varies along the length of the strip, being lower in the regions adjacent the tabs than elsewhere. In such regions the strip is effectively wider on account of the tabs, and electric current flows though the tabs in addition to the strip, so resulting in the lower current density referred to above.
Strip-type heating elements are known, for example, from GB-A-1 569 588, U.S. Pat. No. 600,057 and U.S. Pat. No. 4,292,504.
According to GB-A-1 569 588 the heating conductor strip is slotted alternately from opposite edges to give the conductor strip a zig-zag form and anchoring tabs extend from the strip and penetrate an insulating sheet to secure the strip to the sheet. The tabs may be notched some distance from the lower edge of the strip to form a bending point. Thus, the heating part of the strip is discontinuous and there is no teaching of providing holes, slots or slits in the tabs to improve the uniformity of radiance of the strip along its length.
According to U.S. Pat. No. 600,057, a heating element may be provided in strip form, but there is no consideration of improving the uniformity of radiance of the strip along its length.
According to U.S. Pat. No. 4,292,504, a heating element of expanded metal is supported on edge substantially along its entire length on a board of insulating material. Because the heating part of the strip is discontinuous and no discrete tabs are provided the problems of uniform radiance are not the same as those encountered with a continuous heating strip.
The term `microporous` is used herein to identify porous or cellular materials in which the ultimate size of the cells or voids is less than the mean free path of an air molecule at NTP, i.e. of the order of 100 nm or smaller. A material which is microporous in this sense will exhibit very low transfer of heat by air conduction (that is collisions between air molecules). Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid. A substantially identical structure can be obtained by controlled precipitation from solution, the temperature and pH being controlled during precipitation to obtain an open lattice precipitate. Other equivalent open lattice structures include pyrogenic (fumed) and electrothermal types in which a substantial proportion of the particles have an ultimate particle size less than 100 nm. Any of these particulate materials, based for example on silica, alumina or other metal oxides, may be used to prepare a composition which is microporous as defined above.
The microporous insulation typically comprises a dry particulate microporous material as defined hereinabove mixed with ceramic fibre reinforcement, titanium dioxide opacifier and, for high-temperature use, a small quantity of alumina powder to resist shrinkage. Such microporous insulation material is described in GB-A-1 580 909.